This invention relates to a semiconductor defects curing method and apparatus in which the performance of the semiconductor device is improved. More particularly, this invention relates to measures for eliminating disadvantages caused by the effect of electrical shorts and shunts due to pinholes or other gaps created during the fabrication process of the device.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor alloys, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n and other type devices which are, in photovoltaic and other applications, substantially equivalent to their crystalline counterparts. As such a device, there are disclosed improved photoelectric cells in Japanese patent published application Nos. Sho 55-4994, 55-124274, 56-13777, 56-13778, and 56-13779.
One example of a prior art photovoltaic device is shown in FIG. 1. In the figure, a transparent conductive film 2 is formed on a glass substrate 1 with a mask aligned above the substrate 1. Semiconductor layers 3 are deposited on the substrate 1 with the conductive film 2 therebetween with a mask aligned above the substrate 1. Further, on the substrate 1 with the conductive film 2 and the semiconductor layers 3, aluminum layers 4 are formed as second electrodes with a mask. Reference numerals 31 and 11 designate respective photoelectric cells in the figure.
The two cells 31 and 11 are connected in series by means of a connection 12. In the connection 12 the second electrode 38 is made to be in contact with the first electrode 37. Although the figure appears only with two such connections, a number of the cells are connected with each other in series. The integrated photovoltaic device tends to be degraded after thermal treatment at 150.degree. C. for tens of hours. The reason for the degradation is attributed to a reaction between the aluminum layer 4 and the semiconductor layer 3. Such a device is not suitable for outdoor use where the device is likely to be subjected to a high temperature ambient.
To eliminate the adverse reaction, use is made of a double-layered electrode as the second electrode, composed of a conductive transparent layer such as an ITO film below the aluminum electrode, the ITO film being free from reaction with the aluminum layer or the semiconductor layer. The conductive transparent electrode, however, tends to be finely deposited throughout the semiconductor including pinholes, gaps, or the like type defects introduced during fabrication processing. The transparent electrode material in the defects constitutes short current paths either as deposited or after fabrication. Because of this, only photovoltaic devices having narrow converting areas such as of 1 cm .times.4 cm are currently available.
One attempt to eliminate short current circuit paths within amorphous semiconductor photovoltaic devices involves the application of a reverse bias voltage to the device. This applied reverse bias causes large currents to flow through the short circuit current paths causing localized heating of the current paths. The localized heating crystallizes the amorphous semiconductor in the region of the short circuit current paths, thereby resulting in an increase in the resistivity of the paths. Unfortunately, this process has many limitations. The resistivity of the path remains less than the resistivity of the unheated amorphous semiconductor device area, even after having been increased by the concentrated current. As a result, the short circuit current paths are not eliminated, but merely have their resistivity changed to a limited extent. Also this method is not effective for eliminating short circuit current paths resulting from substrate surface irregularities which can be a most prevalent cause of short circuit current paths, especially in large area devices having a roughened substrate surface forming a diffuse back reflector.